Titanium Bullets, Rocket Sleds, and C-4: How the U.S. Tested the Safety of Nuclear Batteries

To explore the solar system’s darkest, deepest, and most frigid territories, nuclear batteries are the best power sources available. There is, however, a downside: The isotope of plutonium inside them (plutonium-238) is some 270 times more radioactive than the isotope of plutonium inside nuclear bombs (plutonium-239).

As a result, anti-nuclear activists consistently protest the launches of NASA’s plutonium-powered probes. They posit that space exploration isn’t worth the risk of catastrophe. But the Department of Energy, which builds the batteries inside NASA’s fleet of deep-space robots, spared no abuses in assessing their safety.

“They wanted proof of the worst that could happen, so we did our best to smash them, blown them up, shoot them and break them,” said Mary Ann Reimus, an impact test engineer at Los Alamos National Laboratory who tested the safety of nuclear batteries throughout the 1980s and 1990s.

Reimus and others carried out most of their tests in remote deserts near Albuquerque, New Mexico while looking on from concrete bunkers. To conserve precious plutonium, researchers simulated the metal using similarly dense depleted uranium, which posed far less of a radiological threat.

Photographs documenting the DOE’s battery testing program are lost in an expansive Indiana Jones-style warehouse without an archivist to locate them, according to a DOE spokesperson. Fortunately, Reimus kept copies of her studies and shared them with WIRED.

In the photo above, a rocket sled at Sandia National Laboratories accelerates a missile to hundreds of miles per hour. Similar rocket sleds smashed, sliced and slammed log-sized nuclear batteries to explore the limits of their safety. Neither the depleted uranium nor limited amounts of plutonium-238 had any chance of causing a runaway nuclear explosion during the tests.

See how the government pushed the limits of its radioactive power sources in the rest of this gallery.

General Purpose Heat Source

At the heart of every nuclear battery launched since Ulysses in 1990 is an aggressively engineered black brick called a general-purpose heat source, or GPHS (above).

The black comes from Fine-Weave Pierced Fabric, a three-dimensional braid of graphite carbon fiber. It’s the most impact-resistant material capable of also resisting the 1,800-degree-Fahrenheit heat generated by plutonium-238, the radioisotope inside NASA’s batteries.

“This stuff holds together like metal and is really hard to pierce or cut it or break,” Reimus said. “It’s like the graphite in lightweight bicycles, but much tougher.”

Each brick contains two screw-top tubes made of a similar carbon-fiber material. Stacked in each of the tubes are two golf ball-sized metallic capsules. They are made of an alloy of iridium and tungsten — two of the hardest-to-melt metals in existence — and are welded around pellets of plutonium-238 fuel.

Despite these safeguards, GPHS units aren't invincible. As a precaution, engineers pressed the plutonium into ceramic form. That way, if the capsules somehow open upon atmospheric reentry, the plutonium will break into chunks too big to be inhaled (where they can do the most damage).

Because plutonium-238 is an exceedingly precious commodity — NASA may have just 36 pounds left today, or barely enough for a few small missions — Reimus and colleagues used pellets of depleted uranium as stand-in plutonium fuel (below).

High-Speed Impact Testing

One of the worst accident scenarios for a nuclear battery is a rocket launch explosion. Such a catastrophe could shoot a battery at incredible speed through a tempest of flying shrapnel and scorching rocket fuel.

During the mid-1990s, Reimus and her team traveled to Sandia National Laboratory in Albuquerque, New Mexico to simulate elements of a launch disaster for the Cassini spacecraft’s nuclear batteries. They used the laboratory’s high-speed rocket sled to slam the batteries into a concrete wall while being sliced edge-on with pieces of aluminum fuselage (above).

“We built a shed full of pretty fancy cameras and lights so we could see what happened during impact,” Reimus said of the recording rig (below). “The tricky part was that they had uranium instead of plutonium inside, so we had to pre-heat them in a furnace and then quickly lower each one onto a sled.” The furnace (below) helped make the tests as realistic as possible, seeing as a cold battery would perform much differently.

“Then we did our worst,” she said.

Mighty Mouse Rocket Motors

Reimus’ team custom-built a sled adorned with “Mighty Mouse” rocket motors (above) to hurry nuclear batteries to their dooms.

For the impact tests at Sandia, Reimus asked NASA’s Kennedy Space Center ship her “a big chunk of their launch pad.” “Then we smashed the heck out of them against it,” she said.

The shell of one nuclear battery traveling at 130 mph was obliterated by the impact (below). The tungsten- and iridium-capsuled fuel pellets it contained were crushed by the wreck (bottom), but held up well enough to contain the faux plutonium fuel.

In a more extreme and less likely impact at 172 mph, the fuel capsules fared worse. Some tore open because they collided with the battery’s ultra-tough titanium skeleton.

The team was testing a nuclear battery design for two spacecraft launched by the space shuttle: Ulysses, a solar probe that launched in 1990, and Galileo, a Jupiter probe that launched in 1995.

To contain any blown-up pieces of a battery, the researchers built a steel testing chamber (above). Most tests kept all of the pieces inside. One test that detonated 110 pounds of C-4, however, shot a fuel capsule right out the back side of the chamber. The capsule survived unscathed.

Titanium Bullets

One setup used a 0.50-caliber gun armed with an aluminum bullet (bottom, top graphic) to shoot the GPHS brick. That test showed a GPHS’ shell can soak up more than one-fourth of the energy of a bullet traveling at 800 mph. For bullets traveling about 3,460 mph, the bricks deflected 10 percent of the energy away from the fueled capsules.

Other tests with 0.30-caliber titanium bullets (below, bottom) showed capsules could survive a direct hit from a small projectile traveling 950 mph.

All in all, Reimus said the way nuclear batteries are built makes them “pretty darn indestructible,” even if they’re not invincible.

“If they were going to fail, we wanted to see that,” Reimus said. “We were mad scientists to do these tests, but not mad enough that we’d want to contaminate the world.”

She continued, “They wouldn’t have launched if we didn’t do this work to show they were safe.”

All images: Department of Energy (Note: Some image processing was used to recover the quality of scanned printed photographs.)